Emacs: Universal Argument (prefix arg)

Emacs has a mechanism for a command to have variant behavior if user calls universal-argument 【Ctrl+u】 first, before calling the command.

The purpose of universal-argument is:

• A convenient way to let user pass numerical argument to a command. (What the command do with number argument depends on the command. Usually repeating the command.)
• To have a alternate behavior of a command. (e.g. copy file name without dir path, or copy full path.)
• If a command is not designed for taking universal-argument, universal-argument has no effect.

digit-argument, negative-argument

These are variant of universal-argument.

• digit-argument is equivalent to calling universal-argument 【Ctrl+u】 then type a number.
• negative-argument is equivalent to calling universal-argument 【Ctrl+u】 then type a negative number.

Keyboard shortcuts for digit-argument

• The keys Ctrl+1 to Ctrl+9 and Meta+1 to Meta+9 are bound to digit-argument.
• The key Ctrl+- and Meta+- calls negative-argument.

Examples of universal-argument

Type Ctrl+u 60 - to insert hyphen, 60 times.

In dired, dired-copy-filename-as-kill 【w】 copies the file name, but type Ctrl+u 0 first, then w, the copied name is file full path.